Op amp gain trouble.

R3 is because i'm using one of those radio-hack board that has the stuff like switches and multimeter and led's and pots and crap all in it already with a breadboard int he middle and the multimeter reads mV's so the r3 10k resistor is meant to drop the voltage reading to the mV area so it doesn't destroy the multimeter on the board. I have a seperate digital multimeter which is what i used ot test for the values.

I have a radio-shack electronics lab with a breadboard in the middle and pots and stuff all around. It includes a multimeter that can only read mV's so i had to include 10k resistor in series with the multimeter to drop the Voltage down to a level the multimeter can safely read. I have a Digital Multimeter taht i used to take the values. The one on the board is analog.

You're forgetting about DC offset errors. A TLC272 is not suited to precision DC amplification.

What's the point of R3 btw ?

Graham

No, the gain is (R1+R2)/R1. Which indeed is 11 in your case.

If you check the datasheet, you'll see that the typical offset voltage of this opamp is 1.1mV. This offset gets added to the input voltage. So in your case, (2.0mV + 1.1mV) * 11 = 34.1mV. Almost exactly what you measured....

Meindert

Please forgive my ignorance. I've downloaded the datasheet of TLC272 from Texas Instrumenents.

On P5 & P6, the VIO for TLC272C are TYP = 1.1mV and MAX = 10mV for both VDD = 10 & 5V.

What is the parameter Rs stood for and how do I make VO=1.4V? P28 does memtion RS on a graph but I couldn't get any clue out of it.

Will blowing hot air onto the chip while operating make the offset voltage raise up to 11mV?

Allen

Yuk. It's like doing some kind of arthroscopic surgery. Why not go to the horse's mouth.. TI's website?

Rs would be source resistance. It's not particularly important for Vio except to notice that it's very low, so as not to include much in the way of input bias current effects.

You make Vo = 1.4V by adjusting the input voltage(s). It's unclear how exactly they do the test. Another version of the data sheet for the same part has this footnote:

• All characteristics are measured under open-loop conditions with zero common-mode voltage unless otherwise specified. Unless otherwise noted, an output load resistor is connected from the output to the ground pin.

It could cause thermocouple voltages that happen to add to existing Vio. You should avoid thermal gradients and fast changes if you want to measure the actual Vio. But the effect will generally be

Perhaps he wanted the old 1994 version b of the datasheet, instead of the newer 2002 version e on TI's website? :-)

It'll also make a thermal gradient across the die, which will shift the Vgs voltage-matching of the input transistors, and the current mirrors. Vgs is a volt or two, so a 10mV difference is only 0.5 to 1% change in Vgs. A quick glance at a few MOSFET datasheets shows the absolute value of Vgs can change 200mV for a 75C temperature change (in the sub-threshold region), so a 10mV mismatch shouldn't be too hard to achieve, with sufficient mistreatment of the part.